How skilled are the people who tied them there, and how big of a hurry were they in? IMO, inexperienced people tend to tie tightly and clumsily.
If one loop is loose, it's a simple matter of twisting your wrist out around it. Then the slack can be used to wiggle out of the rest.
When you're tying someone's wrists, it's important to keep the knot well away from their fingers. A person can often sort of work their hands inside the bonds until they can reach the knot. An inexperienced villain (or a person in a hurry) might not realize that your characters can reach the knot.
Maybe your characters can't reach their own knots, but one can reach the other's?
Maybe the ropes are tied too tightly. Circulation gets cut off. The heroes convince a guard to loosen the bonds. Guard grumbles but checks anyway, sees the heroes' hands are purple, swollen, and cold. An escape attempt happens or something?
I remember reading one of the Redwall books when I was a child. One of the heroes gets captured and tied, then left on a beach to drown over a course of days. As the water rises, the rope swells and tightens. The tide goes down before they drown, and the rope dries out. The rope is now loose enough to wiggle out of. I thought that particular escape was clever.